Thin-film crystalline silicon solar cells
Kenji YAMAMOTO

1. Introduction
With raw materials for crystalline solar
cells in increasingly short supply, renewed efforts are being made in the study of ways tomake practical thin-film solar cells for use intomorrow’s large-scale solar cell installations.
In particular, since thin-film polycrystalline silicon solar cells are made of the same materials
as bulk silicon solar cells which are currently
in widespread use, they are being enthusiastically studied at laboratories all over the world
due to the abundance of raw materials forsuch devices, their stability, and the characteristics they are expected to possess as silicon
devices. Part of the motivation behind these

750
700

open circuit voltage Voc (mV)A photoelectric conversion efficiencyof over 10% has been achieved in thin-film
polycrystalline silicon solar cells which consistsof a 2 µm thick layer of polycrystalline siliconwith a very small grain size (microcrystalline
silicon) formed by low-temperature plasma
CVD. This has shown that if the recombination velocity at grain boundaries can be madevery small, then it is not necessarily important
to increase the size of the crystal grains, andthat an adequate current can be extracted
even from a thin film due to the light trapping effect of silicon with a low absorption
coefficient. As a result, this technology may
eventually lead to the development of lowcost solar cells. Also, an initial efficiency ashigh as 12% has been achieved with a tandem solar cell module of microcrystalline silicon and amorphous silicon, which has now
started to be produced commercially. An evengreater initial efficiency of 14.5% has beenachieved in devices with a small size area,and further increases of efficiency can be expected.
Keywords: solar cell, silicon, light trapping,thin film, microcrystalline, plasma chemical vapor deposition,
recombination velocity

JSAP International No.7 (January 2003)

Astropower

Mitsubishi

600
550
500

ASE/ISFH
Ti
Daido

Sanyo

KanekaNeuchatel
103

104

450

10-2

10-1

ISE

ETL

105

106
BP

400
350

100

S=107cm/s
Tonen

101

102

103

104

grain size � ( µm)
Fig. 1: The relationship between grain size and open circuit voltage (Voc) in solar cells.
Voc is correlated to the carrier lifetime (diffusion length). In the figure, S indicates therecombination velocity at the grain boundaries. In this paper, microcrystalline silicon cells
correspond to a grain size of 0.1 µm or less. In the figure, Ti, BP, ASE and ISE are abbreviations of the research facilities from which the associated data came.

studies comes from the theoretical findingthat if these devices can be constructed sothat they trap sufficient light, it should ideallybe possible to achieve photoelectric conversion efficiencies in excess of 20%, even withsolar cells in which the photoelectric layer isjust a few µm thick.1)
Historically, the silicon thin-film photo absorbed layers in such devices have been produced by high-temperature processes, mainly
chemical vapor deposition (CVD), 2) liquid
phase epitaxy (LPE),3) and zone melt recrystallization (ZMR).4)
However, it has become apparent thatthe properties of the underlying substrate
are the key to obtaining high-performancesilicon thin-film solar cells. That is, a highquality silicon thin-film photoelectric layer
can be obtained by suppressing the diffusionof impurities from below and using epitaxial
deposition that exploits the characteristics of
the underlying material, and evidently single-

PV Research Division, Kaneka Corporation (2-1-1, Hieitsuji, Otsu 520-0104)
e-mail: yamamoto@pv.kaneka.co.jp

12

650

crystal silicon has become the optimal material for the substrate. Accordingly, this research
has shifted toward recycling single-crystal silicon, and at present research is being actively
performed into single-crystal silicon thin-film
solar cells, primarily focusing on porous silicon delamination techniques.5)
On the other hand, attention has recently
been focused on thin-film solar cells madewith crystalline silicon having a small grain
size formed by plasma CVD using inexpensive
substrate materials such as glass and by using
low temperatures regardless of the type of
substrate instead of these high-temperatureprocesses.6–11)
Figure 1 shows the relationship between
grain size and V oc (open circuit voltage) assummarized by J. Werner of Stuttgart University.12) Here, Voc can be regarded as a parameter reflecting the cell characteristics and crystalline properties. As the figure shows,superior characteristics are obtained with agrain size of 100 µm, but the characteristics
are worse with a grain size of a few tens of µ
m. Conversely, favorable characteristics havebeen obtained experimentally both at Neuchâtel
University and at Kaneka Corporation’s PV Research Division by using thin-film polycrystalline
silicon formed at low temperatures by plasma
CVD with a submicron grain size (this is generally
referred to as microcrystalline silicon due to thesmall grain size). This is an interesting discovery
which has caused recent research to shift towardboth extremes.
This report discusses these thin-film polycrystalline silicon solar cells formed at low
temperature (referred to as microcrystalline
silicon solar cells below) which have been thesubject of studies conducted by the PV Research Division using thin-film polycrystalline
silicon with a very small grain size formed at
low temperature. It will also introduce the results of the research and development of silicon hybrid solar cells with a tandem structure
comprising these solar cells and amorphous
solar cells, which were put to practical use forthe first time in 2001.

to light. The very first reports on the characteristics of an n-i-p cell were made by the
PV Research Division using the light trapping
structure described below. These cells had an
intrinsic conversion efficiency of 10.7% andan apparent efficiency of 10.1% for a film
thickness of 2 µm (surface area 1 cm2, measured by the JQA (Japan Quality Assurance
Organization)) (Fig. 2).8–11) Also, by subjecting the silicon film in the photoelectric layerof this cell to XRD (x-ray diffraction) measurements, it was found to have a preferential(110) orientation. The p-i-n and n-i-p cells
have different characteristics due to their different fabrication sequences. A large difference is that the underlying layer of a p-i-n cellis the transparent p-type electrode, whereas the underlying layer of a n-i-p cell is then-type back electrode.
As a general rule, transparent electrodes
are made of oxides, and since there is a riskof these oxides being reduced by the hydrogen atoms that are needed to form microcrystalline cells, there is a smaller process
window in the cell formation conditions forthe p-i-n type. From the viewpoint of theease with which integrated structures can beformed, which is a characteristic of thin-film
solar cells, an advantage of p-i-n cells is thatthey can be formed as super straight modules
using integration techniques similar to thoseused for amorphous silicon as described below. On the other hand, it should be possible to make integrated structures of n-i-pcells by methods equivalent to those used forCu(InGa)Se2-based solar cells.13) At the present time it is difficult to determine which is
better, but it should become possible to arrive
at a conclusion in terms of cost, performanceand applications through the production of
sub-modules in the future.

2.2 Carrier transport in microcrystalline silicon thin-film solar cells
To improve the efficiency of these solar
cells in the future, it is absolutely essential togain some understanding of the relationshipbetween the crystalline microstructure including grain boundaries and the carrier transport
process, in particular the carrier lifetime (diffusion length) that determines the solar cellcharacteristics and the recombination velocity

2. Thin-Film Polycrystalline(Microcrystalline) Silicon Solar
Cells
0.035

2.1 Microcrystalline silicon thinfilm solar cells formed at low temperature

0.020

Jsc : 25.8 (mA/cm2): 24.35 (mA/cm2) (AP)Voc : 0.539 (V)F.F. : 76.8 (%)
Eff AP : 10.1(%)
Eff IN : 10.7 (%)

0.020

0.015

0.015

0.010

POWER (W)

0.025

CURRENT (A)Microcrystalline silicon solar cells formedby plasma CVD at low temperature are assumed to have a shorter lifetime than singlecrystal cells, and it is common to employ ap-i-n structure including an internal electricfield in the same way as an amorphous solar cell. A p-i-n type microcrystalline siliconsolar cell is formed by a process fairly similar
to that of an amorphous solar cell. Strictly
speaking, these cells can be divided into p-i-n
and n-i-p types according to the film deposition order, although the window layer of thesolar cell is the p-type layer in both cases.
The characteristics of a cell having a p-i-nstructure were first reported by Neuchâtel
University.6,7) Unlike an amorphous solar cell,
this cell does not deteriorate when exposed

0.030

0.010
0.005
0.005
0
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0

VOLTAGE(V)Fig. 2: Current-voltage characteristics of an n-i-p type microcrystalline silicon cell (film thickness
2 µm, area 1 cm2, measured by JQA). In this figure, EffAP is the characteristic for the area including the grid electrode, and EffIN is the characteristic of the effective area not including the grid.

JSAP International No.7 (January 2003)

13

AFM:
Surface roughness
Sq=40nm Sq=18nm

Sq=17nm

Sq=16nm

Sq=4nmTEM:
Crystalline size and structure
24nm

28nm
9nm

10nm

750nm
300nm

30nm

200nm

1.25

2.5

5

7.5

8.6

(SiH4) / (SiH4+H2)[%]
Fig. 3: The variation of crystalline structures in microcrystalline silicon as a function of SiH4 gas
concentration according to the results of measurements by TEM (transmission electron microscopy) and AFM (atomic force microscopy). The small black dots represent the amorphous state.
Sq represents the surface roughness (r.m.s. roughness).

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Thickness( µ m)
Fig. 4: The variation of Voc with film thickness. In this figure, the open circles represent experimental data, Pfi(%) represents the confinement factor (0%, 70% or 92%), L represents the diffusion length, and Ni represents the carrier concentration. A simulation was also performed using
PCID.16) The simulated results agree well with the experimental data at a carrier concentrationof 1015/cm3.

at grain boundaries. However, in a microcrystalline silicon cell, the general non-uniformitythat is typical of its grain structure has a direct and complex effect on carrier transport,
and thus there have been no conclusive reports relating to these physical properties bydirect measurement and analysis. Rather, theefficiency of these solar cells has only recently come to light, so only the first steps havebeen made in the research of their funda-

14

JSAP International No.7 (January 2003)

mental properties.
A report by Werner et al. 12) discussed
why the very small crystals produced at low
temperature (as discussed in the introduction)
exhibit relatively favorable cell characteristics. Although the cell characteristics are theproduct of current, voltage and fill factor, theopen circuit voltage (Voc) is directly related tothe cell’s lifetime and is used here as a guideto the overall cell characteristics. This is be-cause the current is related to light trapping
effects and the cell film thickness as discussed
below.
According to Werner et al., the reason
why a microcrystalline silicon cell exhibits acharacteristic of 10% or more regardless of
its small grain size is because of the very low
recombination speed at its grain boundaries. This low recombination velocities originates from the small barrier height at thegrain boundaries, which is guaranteed by a)the passivation of hydrogen at grain boundary defects, b) uncharged intrinsic grains (lowoxygen density), and c) the (110) orientationof the crystal grains.
In Fig. 1, as can be seen from the relationship between the open circuit voltage(V oc), the grain size and the recombination
velocities already summarized by Werner etal., further improvement of the lifetime of
the crystalline silicon layer (i.e., improvementof Voc) in order to make substantial efficiency
improvements may require not only a substantially larger grain size but also a lower recombination velocities at the grain boundaries (e.g., 100 cm/s). Or to put it another way,
however much the grain size is increased asshown in Fig. 1, the conversion efficiency decreases when the recombination velocity at
the grain boundaries increases (corresponding
to a film produced at high temperature).
The actual crystalline structure of a microcrystalline silicon solar cells has been investigated by TEM (transmission electron
microscopy) and AFM (atomic force microscopy). For example, Meier et al. at Neuchâtel
University have classified the crystalline structures as shown in Fig. 3. Of course the microstructures differ depending on the production process and conditions. It has also beenreported that diffusion transport mechanisms
are dominant within the grains whereas drift
transport is more prominent at the grainboundaries.14) These grain boundaries exhibit
amorphous characteristics which give rise toa passivation effect, and it has been pointed
out that this may reduce the boundary recombination velocities.
At the Julich Research Laboratory, Rech etal. evaluated the cell characteristics as a function of the silane concentration with respect
to the hydrogen dilution,15) and showed thatthe peak cell performance (where the maximum value of Voc is achieved for the crystal)occurs at the boundary where the resulting
film changes from amorphous to crystalline.
It is hoped that a more precise analysis of the
microstructure and a deeper understandingof its relationship with the boundary recombination velocities will be possible in the future.
To understand the phenomena behind
carrier transport, the PV Research Division isat the initial stages of a study into the relationship between Voc and film thickness.10,11)
Figure 4 shows the dependence of V oc onfilm thickness for a microcrystalline silicon device we produced. The maximum value of Voc
we obtained was about 550 mV. As the film
thickness decreases, Voc increases slightly. Thisis thought to be because as the film thickness
decreases, the overall number of defects decreases and Voc increases. We thus obtained
a result that agrees with the theory stated at
the beginning that Voc improves with a thinner film.
This figure also shows the results of simulation by one-dimensional PCID.16) For a given carrier lifetime (for the case of diffusion
length), V oc can be increased by further increasing the carrier concentration. However,according to Werner’s findings, this causes
the grain boundaries in microcrystalline layers to become weakly n-type, resulting in ahigher recombination velocities. This has theeffect of causing Voc to decrease, so it is alsoessential to control the grain boundaries.
In any case, compared with the V oc ofhighly efficient single-crystal solar cells (about
700 mV), the V oc of microcrystalline siliconsolar cells is at least 20% lower, and the key
to improving the cell efficiency is to increase
Voc—i.e., to improve their lifetime.

microcrystalline silicon by plasma CVD arevery simple—as long as it is possible to supply a sufficient quantity of hydrogen atoms
to the substrate during crystallization withoutcausing any damage (most of it is ion damage), then the film fabrication rate can be
improved without deteriorating the cell characteristics. Throughput is another importantconsideration for mass-production, and it istherefore essential to develop ways of depositing films at high speed over large areas. At
the PV Research Division, as stated below, wehave devised a technique for fabricating uniform thin films of microcrystalline silicon withhigh quality at a film deposition rate of 1.1
nm/s on meter-size substrates.

2.4 The light trapping effect of microcrystalline silicon cells
Light-trapping techniques are a way of
increasing the performance of microcrystalline solar cells. This is a core technique forcells made from microcrystalline silicon because—unlike amorphous silicon—it is essentially an indirect absorber with a low absorption coefficient. That is, the thickness of
the Si film that forms the active layer in a microcrystalline silicon solar cell is just a few µm,
so it is not able to absorb enough incident
light compared with solar cells using ordinary�����Surface
Texturep������
i��������
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2.3 High-rate fabrication of microcrystalline silicon films
To consider the fabrication rate of microcrystalline silicon films, it is essential tounderstand the film fabrication mechanisms
involved. A detailed description of thesemechanisms can be found in a report published by Kondo et al.17)
But based on the large amount of experimental data gained by the PV Research Division, the guidelines for forming high-quality

crystalline substrates. As a result, it is difficult
to obtain a high photoelectric current. Lighttrapping technology provides a means of extending the optical path of the incident lightinside the solar cell by causing multiple reflections, thereby improving the light absorption in the active layer.
Light trapping can be achieved in two
ways: (1) by introducing a highly reflective
layer at the back surface to reflect the incident light without absorption loss, and (2)by introducing a textured structure at theback surface of the thin-film Si solar cell. Of
course, the carrier lifetime is also important,and it goes without saying that the diffusion
length must be at least as long as the film
thickness.
At the initial stages, we focused on thelight trapping effects of textured structures
formed on the silicon surface.8) We then improved the solar cell characteristics by reducing the film fabrication temperature (although
as the film fabrication temperature decreases,
the degree of surface texturing also decreases). A solar cell with a textured surface and aback reflective layer for increasing absorption
is said to have a STAR (naturally Surface Texture and enhanced Absorption with a backReflector) structure.
At present, two types of light trapping

Back
reflector
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(b) 2nd generation���������������������������
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Fig. 5: Cross-sections through light-trapping microcrystalline silicon solar cell devices. (a) First generation (flatback reflector); (b) Second generation (textured back reflector, thinner polycrystalline silicon layer).

JSAP International No.7 (January 2003)

15

16

JSAP International No.7 (January 2003)

E.Q.E.

Reflectance

E.Q.E.

texture characteristics depend on the fabrication conditions as well as the film thickness.
100
Cells of this sort are fabricated as follows.
First, after a back reflector layer has been
80
formed on a glass substrate, a layer of n-type
60
silicon is deposited on it by plasma CVD.
Internal reflectance
Next, the active layer, which is a thin film ofP
fi40
intrinsic polycrystalline silicon, is formed by
92 (%)
70 (%)
plasma CVD. Finally, a p-type silicon film and20
0 (%)
an ITO (indium tin oxide) film are depositedExperimental
0
in turn, and then a comb grid electrode is
400
500
600
700
800
900
1000 1100
formed on top.
Wavelength (nm)Figure 6 shows the results of investigatModel Parameters
ing the collection efficiency of cells with a 4.7
Pe
thickness : 4.7 µ mµm thick active layer. In this figure, the open
Ldif : 7 µm
Phcircles indicate measured results. At longer
Ni : 1016cm-3
Pfi
wavelengths, this cell exhibits excellent senPh : 95%
sitivity characteristics, and under an AM1.5
light source it was found to generate a current density in excess of 27 mA/cm2.10,11) The
solid, dotted and dashed lines in the figure
Fig. 6: Absorption efficiency of a 4.7 µm cell. In this figure, the open circles indicate
measurement results. The solid, dotted and dashed lines show the spectra calculated usshow the spectra calculated using a PC1D.16)
ing a PC1D.16) The surface internal reflectivity Pfi (indicating the ratio of trapped light)
These are the results obtained by changing
was 0, 70 and 92% respectively, In the figure, Pe represents the surface reflectivity, and Ph
represents the back surface reflectivity.
the surface internal reflectivity Pfi to 0, 70 and
92% respectively, and are used as a parameter to express the light trapping ratio of thestructure are in use. Figure 5(a) shows a cell
this texture is strongly dependent on the filmcell. A value of 95% is assumed for the backthat uses a flat highly reflective layer at the
thickness. When the film is relatively thickreflection rate. The other parameters are asback surface, and Fig. 5(b) shows a cell that
(4 µm or more), the surface texture is suitshown in the Figure.
uses a textured type of highly reflective layer.
able for light trapping, but when the film is
Changing the surface internal reflectivThis highly reflective back layer also acts as
relatively thin (1.5 µm or less), an adequate
ity causes the spectra to change in the wavethe solar cell’s back electrode. Thin-film polysurface texture does not form.8–11) It is thislength region above 600 nm where the light
crystalline silicon forms naturally with a texnecessary to use a textured reflective layer at
is reflected from the back surface of the cell
tured structure on its surface, and the size ofthe back surface. Of course, to be precise theand surface effects are apparent. As the surface internal reflectivity increases, the sensitivity to infrared light increases. Fromthe experimental results
1.0
1.0
and calculated results, the
0.8
0.8
surface internal reflectivity
Flatof this cell corresponds to
Texture
0.6
0.6
70%.
Now consider the light
0.4
0.4
trapping effect of a thin film
Texture0.2
0.2
polycrystalline silicon layer
Flatwith a thickness of just 1.5
0.0
0.0
400
600
800
1000
1200
µm. Figure 7(a) shows the
400
600
800
1000
Wavelength (nm)reflection spectra of samWavelength (nm)ples corresponding to two(a)(b)
types of 1.5 µm thick cell(one with a flat back reflecFig. 7: The characteristics of microcrystalline solar cells on glass substrates having flat and textured back reflectors. (a) Reflectivity, and (b)tor and one with a textured
collection efficiency characteristics (quantum efficiency).

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Fig. 8: A next-generation tandem
model. (a) Device cross-section of anamorphous – microcrystalline siliconlayered next-generation tandem cellhaving a transparent intermediate layer (next-generation silicon hybrid cell);(b) Spectral sensitivity characteristicsof the hybrid cell including the intermediate layer (absorption efficiency).

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back reflector), and Fig. 7(b) shows the corresponding collection efficiency spectra.
In Fig. 7(a), the minimum that appears
in the spectra at around 550 nm arises because the ITO film was deposited under nonreflective conditions. The oscillation seen at
wavelengths above 600 nm is caused by interference with the silicon film in the active
layer. The reflectivity of the sample with thetextured back surface is significantly lower in
the infrared region. Figure 7(b) shows the
spectral sensitivity spectra corresponding to
the flat and textured samples. In the case of
the textured sample, it can be seen that thesensitivity is higher at wavelengths above
600 nm, which corresponds to the reductionof the reflectivity spectrum at longer wavelengths.
This result provides experimental verification of the contribution made by minutetextured structures to light trapping. In the
future, it will be necessary to aim at increasing the efficiency by obtaining larger currents
in the thin film through the formation of better light trapping structures by controlling the
profile of the microcrystalline silicon surface
and the underlying reflective layer.

3. Application to Stacked (Hybrid) Type Solar Cells
3.1 Silicon hybrid solar cells
Although the microcrystalline siliconcells formed at low temperature as de-��������
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scribed above have a potential for high efficiently, their efficiency in single-cell structures is currently only about 10%, which ismuch lower than that of bulk polycrystalline
cells. To achieve a substantial improvementof efficiency there needs to be some kind ofbreakthrough, such as a substantial reduction in the abovementioned grain boundary
recombination or the establishment of moreadvanced light trapping techniques. In an attempt to achieve this, we have investigated
the use of two- and three-stacked (hybrid)
structures in which multiple cells with different light absorption characteristics arestacked together. This approach allows better characteristics to be obtained with existing materials and processes. The advantagesof using a layered structure include the following: (1) it is possible to receive light bypartitioning it over a wider spectral region,
thereby using the light more effectively; (2)it is possible to obtain a higher open-circuit
voltage; and (3) it is possible to suppress tosome extent the rate of reduction in cell performances caused by optical degradation
phenomena that are observed when using
amorphous silicon based materials.
At the PV Research Division, we have recently made further progress with the investigation of stacked (tandem) cells in which theabovementioned amorphous silicon cells are
combined with microcrystalline silicon cells,
and have been studying structures in which a

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���transparent intermediate layer is provided between the amorphous silicon layer of the top
cell and the microcrystalline silicon layer ofthe bottom cell (next-generation hybrid solarcell).
Figure 8(a) illustrates the structure ofsuch a device. Compared with a conventional
hybrid cell, the presence of a transparent intermediate layer, by its nature, causes someof the light that reaches the microcrystalline
silicon layer of the bottom cell by passing
through the amorphous silicon layer of the
top cell to be reflected back into the top cell(amorphous silicon). As a result, the effective
light sensitivity of the top cell is higher thanthat of a top cell with the same thickness in aconventional hybrid structure, so it is possible
to expect an improvement in the short-circuitcurrent J sc of the overall hybrid cell. Figure8(b) shows how the sensitivity spectrum of
the hybrid cell is affected in practice by theaddition of a transparent intermediate layer.
Even if the amorphous silicon and microcrystalline silicon layers have the same thickness,the cell with a transparent intermediate layer
is more sensitive at shorter wavelengths (topcell).
This result seems to suggest the possibility of controlling the respective light sensitivities of the top and bottom cells by introducing a suitable transparent intermediate layer.
From the results of Fig. 8(b) it can be inferred that by controlling the thickness of the

JSAP International No.7 (January 2003)17������������

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Fig. 9: Current-voltage characteristic of an amorphous – microcrystalline silicon stacked nextgeneration hybrid cell with a transparent intermediate layer (initial efficiency; area 1 cm2).

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Fig. 10: A silicon hybrid cell. (a) Cross-sectional structure of the silicon hybrid module (amorphous– microcrystalline silicon tandem solar cell module; series-connected). (b) Current-voltage characteristics of a large area (910×455 mm) silicon hybrid module (initial efficiency).

18

JSAP International No.7 (January 2003)

transparent intermediate layer it is possible toreduce the thickness of the top cell needed
to obtain the same current compared with aconventional hybrid cell. As a result, it should
be possible to obtain roughly the same shortcircuit current Jsc even when the amorphoussilicon film thickness is reduced. If the thickness of the amorphous silicon film in the top
cell can be reduced, then it should also be
possible to reduce the photo-degradation ofcell.
This structure (amorphous silicon – transparent intermediate layer – microcrystalline
silicon) thus makes it possible to improve not
only Jsc but also the cell characteristics compared with a conventional hybrid cell with notransparent intermediate layer without reducing Voc or the fill factor (FF). As Fig. 9 shows,an initial conversion efficiency of 14.5% has
been achieved with a 1 cm square small area
cell of this type.18)

3.2 Large area thin film silicon hybrid modules
At the PV Research Division, we havebegun developing techniques for depositing microcrystalline silicon films on large area
substrates, and for the production of silicon
hybrid modules based on tandem cells of
amorphous silicon and microcrystalline silicon.19,20) We have also been developing mass
production techniques, which were put into
commercial operation in 2001. These hybrid
modules employ a super-straight structure inwhich light is incident through a glass substrate on which a transparent electrode isformed, and are integrated by laser scribe
processing (Fig. 10(a)). By adding a microcrystalline silicon film deposition process, theamorphous silicon module production process can be used to fabricate silicon hybrid
modules.
As a result of improving the efficiency ofa large area hybrid module measuring 910
×455 mm, we have achieved an initial efficiency of 12.0% and a peak output power of
46.0 W as shown in Fig. 10(b). We have alsoachieved the goal of a film deposition rate of
1.1 nm/s on metro-size large area substrates.
The efficiency of these cells is somewhat lower (11.2%), but still of practical use.21) Someof these modules are shown in Fig. 11.

a-Si/poly-Si Hybrid Module
910 910mm sized substrate

Field test of
hybrid modules

Fig. 11: Appearance of the hybrid modules.

4. Conclusion
It has been demonstrated that microcrystalline silicon with a very small grain size
formed at low temperature by plasma CVD
is a promising material for use in solar cells.
Amorphous solar cells and stacked silicon (hybrid) solar cells have already been put to practical use in power modules. Further study isneeded to ascertain and control the relationship between recombination at grain boundaries and the grain boundary microstructure,
which is a non-uniform system. This shouldmake it possible to improve the inherent performance of microcrystalline silicon solar cells.
There is also a need for the development ofcrystal deposition methods that allow somedegree of control over the texture structureof the silicon surface to allow a suitable levelof light trapping to be implemented.
This study was partly funded by the New
Energy and Industrial Technology Development Organization (NEDO) as part of the
New Sunshine Project, and we would like tothank everyone concerned.

Kenji Yamamoto
Joined Kaneka Corporation in 1983 on completing a master’s course in engineering at Kyoto University. From 1988
to 1990 he was a guest researcher at Stamford University,and in 1991 he was awarded a D. Eng. Since 1991 he has
been engaged in the study of silicon thin-film solar cells.
A member of the American Physical Society (APS) and theInstitute of Electrical and Electronics Engineers (IEEE).

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(Received January 8, 2002)

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